Field of the Invention
Embodiments of the invention are generally in the field of controlling and/or positioning a physical payload in an unstable medium (e.g., air, water) and, more particularly relate to a method and apparatus for controlling and/or positioning a payload in an unstable medium and compensating for heave or other uncontrolled motion induced by the medium, (e.g., marine wave action), and applications thereof.
Description of Related Art
Heave compensation refers generally to systems that adjust for or otherwise compensate for the motion of a surface ship on equipment suspended overboard in a water column, lifted or lowered through the water column, and or landed on the ocean bottom, a surface platform or dock, or another vessel. In all these cases, the motion of the surface ship induced by wave action acting on it are substantially conveyed, or in some cases amplified and conveyed, to payloads suspended from the ship by rope, cable, chain, belt or similar connecting medium whether flexible or rigid.
FIGS. 1 and 2 illustrate examples of the problem heave compensation systems are intended to address. A surface ship 1 having a deck 10 floats above a surface of a body of water indicated by a waterline 2. The deck 10 is elevated above the waterline 2 and machinery is affixed to it. A crane 40 or similar lifting mechanism is configured so as to be able to lift overboard a payload 60 and raise or lower that payload 60 by rope or cable 30 connected on one end to the payload 60 and on the other end to a winch 20. The cable 30 passes over an overboard-sheave 50 where the direction of the cable 30 is changed from near horizontal to vertical. When at rest, the tension in cable 30 is nominally equal to the weight of the payload 60 plus the weight of the cable 30 between overboard-sheave 50 and payload 60.
In FIG. 2, the ship 1 is raised by wave action above a reference line 100 which it was earlier below as shown in FIG. 1. This happens over a finite period of time wherein the ship 1, and more specifically, the overboard-sheave 50, is accelerated upward. The ship 1 resists this acceleration by settling deeper in the water as indicated by the waterline 2 nearer the deck. The payload 60 also resists this acceleration because of gravity acting on its own mass plus the drag force of the water acting on the payload 60 once in motion. The tension in cable 30 is thereby increased until the vertical velocity of the payload 60 is equal to or exceeds the velocity of overboard-sheave 50. The increased tension in cable 30 can be extreme and introduces loads on all components of the system including the deck 10, the winch 20, the crane 40, the overboard-sheave 50, as well as the payload 60. The entire system must be engineered to withstand the forces that will act on it given a particular sea state defining the safe operating window; otherwise, one or another system component will fail, endangering the mission, equipment, personnel, and/or payload.
When the upward motion of the ship 1 decelerates and subsequently begins to fall back to or through its starting position, all the forces and tensions are reduced but the danger of a mechanical failure is not gone, just delayed until that motion stops. The same drag forces on the payload 60 that worked with gravity to resist its upward movement also act against the payload 60 falling as quickly as gravity alone would cause it to fall. It is in fact possible that overboard-sheave 50 may fall more quickly than the payload 60. This would allow tension in the cable 30 to fall to zero and slack to accumulate in one or more portions of cable 30. In this circumstance the payload 60 is accelerating downward resisted only by its drag in the water and not from any tension earlier supporting it from above by the cable 30. When the downward motion of overboard-sheave 50 ends and is subsequently reversed, the cable 30 will come taut in a “snap load” event. Snap loads can easily exceed the breaking strength of cable 30 and/or the rated operational capacity of other mechanical components of the system. Breakage of the cable 30 and/or damage to other components of the system may result in loss of the payload 60, loss of time and money, as well as cause injury or death.
Heave may be defined as the vertical motion of the overboard-sheave 50 induced by wave action on the vessel, and heave compensation systems are employed to minimize the effects described above thereby widening the safe operating window for the vessel and its machinery in carrying out its mission.
FIG. 3 illustrates a conventional example of a passive heave compensation system that is entirely spring based. It is passive because once engaged, it requires no extra energy beyond the energy introduced into the system by the motion of the ship and payloads themselves. Deck 10, winch 20, cable 30, overboard-sheave 50, and payload 60 are as illustrated in earlier figures. Overboard-sheave 50 is supported by crane 40 (not shown) as before. Two sheave-blocks 70 and 80 are separated from each other by a spring 90. Sheave block 70 is fixed in place, and may be referred to as a “fixed sheave-block”, while sheave-block 80 is movable, and may be referred to as a “flying sheave-block”. The flying sheave-block 80 optionally moves vertically inside a support structure (not shown) that keep it stably centered over the fixed sheave-block 70. As illustrated, the spring 90 is substantially vertically oriented with sheave-blocks 70, 80 aligned one above the other, but horizontal arrangements are possible and common. Cable 30, which in earlier figures passed from winch 20 directly over overboard-sheave 50, instead first makes a complete path around both the fixed sheave-block 70 and the flying sheave-block 80 before making its way over the overboard-sheave 50. One complete path around both sheave-blocks 70, 80 is illustrated but multiple passes, typically 2 (mechanical advantage of 4), are often employed so that shorter excursions of the flying sheave-block 80 can accommodate longer heave excursions at the expense of a stronger spring. Other sheave arrangements are possible and easily comprehended by those skilled in the art.
FIG. 3A shows the reaction of machinery in FIG. 2 to an upward heave event. The upward heave A increases tension on the cable 30 and causes the spring to be compressed, reducing the distance between the sheave-blocks B, and freeing some portion of cable 30 that passes around the sheave-blocks to be released as illustrated. During a downward heave event A shown in FIG. 3B, reduced tension on the cable 30 will allow the spring to expand, increasing the distance between the sheave-blocks B, which in turn takes up what might otherwise be slack in rope 30. One can see that the spring constant must be matched to the load, which includes the payload 60 plus the weight of cable 30 between overboard-sheave 50 and the payload 60. If friction is ignored, the passive system just described is closely analogous to a spring 70 inserted in rope 30 between overboard-sheave 50 and the payload as illustrated in FIG. 4.
In practice, it is not practical to change coil springs based on the mass of the load being handled. Springs in passive heave systems as described are instead “gas springs,” and the typical components are illustrated in FIG. 5. A gas spring 200 consists of a piston 210 free to move inside a piston housing 220, with a bottom seal 230. The piston has seals 211 that prevent gas from passing between the piston 210 and piston housing 220. At the bottom of the piston housing 220 there is plumbing that allows gas to pass freely between the piston assembly 239 and an accumulator 240. The volume inside the piston housing 220 below the piston seals 211 together with volume inside the accumulator 240 constitutes a pressure vessel. The volume of the pressure vessel is further increased by plumbing in a series of gas bottles 250. The gas is typically nitrogen or air, but other gases may be utilized. As the piston 210 is advanced into the piston housing 220, the gas beneath the seals 211 is displaced and therefore compressed uniformly inside all the components making up the pressure vessel. Neglecting well understood details regarding temperature and non ideal gases, the spring constant of the system is adjusted by varying the pressure inside the gas filled portion of the gas spring 200 relying on Boyles Law, where pressure p multiplied by volume v is a constant. The fully pneumatic spring of FIG. 5 represents a passive heave spring but typically a combination gas-over-fluid spring, as illustrated in FIG. 6 is used for reasons unimportant to this discussion. In such springs, the piston housing 220 is filled with fluid 241 beneath the piston seals 211, as is a substantial portion of the accumulator 242 and the plumbing 235 connecting the two. When the piston 210 is advanced into the piston housing 220, instead of compressing gas directly, it displaces hydraulic fluid into the bottom of the accumulator. The gas-fluid interface 243 is inside the accumulator 240. As the level of fluid in the accumulator 240 is increased, it compresses the gas in the upper portion of the accumulator 240 and the remainder of the pressure vessel in just the same manner that the piston itself would in the all pneumatic version of FIG. 5.
The spring constant in a gas spring is easily adjusted by changing the pressure in the pressure vessel.
FIG. 7 shows the principle components of a gas spring in a passive heave compensation system as discussed. The system illustrated and discussed herein above had a single pneumatic or hydraulic piston, but there can be more than one piston (often two) between the flying sheave-block 80 and the fixed sheave-block 70 usually feeding the same accumulator 240.
Passive heave compensation systems based on gas springs are widely used, simple, and very effective at insulating cable 30 from extreme fluctuations in tension. However the spring only responds to changes in the tension of rope 30 at the overboard sheave 50, and any change in this tension will cause the payload 60 to be displaced vertically in the water column. That tension is nominally equal to the weight in water of the payload 60 plus the weight in water of the rope 30 between the overboard sheave 50 and the payload 60. This can be defined as “active-load” and is a largely invariant physical property of payload 60, rope 30, and the earth's gravity. Absent heave, the weight-on-sheave (WOS) at the overboard sheave 50 will nominally be equal to the active-load. However, the WOS is sensitive to heave due to the payload's inertia and the drag forces acting on the payload 60 and rope 30. If the WOS at overboard sheave 50 exceeds the active-load, the payload 60 will be lifted in the water column. And if the WOS at overboard sheave 50 is below the active-load, the payload 60 will fall in the water column.
In addition, the spring cannot respond until the differential tension is sufficient to overcome the friction in the system components, which can be significant. There is substantial friction a) between the seals 211 and the piston housing, b) in the sheaves turning on their shafts, which is increased with increased active-load, and c) between the flying sheave-block and its support structure (if used; not shown) that constrain its motion. Added to the friction in the machinery, cable 30 is likely a relatively large wire rope, synthetic rope, or armor shielded umbilical. Such ropes and cables do not bend easily over a sheave and once bent, resist counter deformation. Added to this is the inertia in all of the massive metal moving parts, which resist being set in motion in the first place, but are particularly resentful of changing direction. Finally, the spring stored energy will be recovered when the heave action is decelerated or reversed. Transmissibility is a well understood property of springs, is frequency sensitive, and is defined as the ratio between output and input amplitude of the spring.
For all of these reasons spring based passive systems are deficient at maintaining a payload 60 stationary in the water column.
When residual motion of the payload 60 is too extreme for the particular mission's purpose, active heave compensation must be employed. Active systems directly control the pay-out and take-up of cable 30 passing over the overboard sheave 50 and/or the elevation of the overboard sheave 50 so as to ideally compensate for the motion of the ship 1, limited principally by the ability to measure and anticipate that motion. Measurement and forecast is typically left to a motion reference unit (MRU) composed of computer, software, and input from various sensors. These systems are complicated and expensive, but even if perfect at measuring and predicting the motion, making real time adjustments in these physical systems (winch 20 start, stop, reverse (FIG. 8)/sheave elevation/crane 40 adjustments (FIG. 9)) typically require substantial additional power (hydraulic or electric) and substantial strengthening of associated machinery, further increasing the cost.
There are active systems that incorporate passive systems as described hereinabove. In these cases, the active system provides power assist (usually hydraulic) to override the motion of the flying sheave-block 80. Such systems are called active-over-passive (AOP) systems as diagramed in FIGS. 10 and 11. FIG. 10 is different diagrammatically but operationally identical to passive gas-spring compensation systems as already discussed. FIGS. 11 and 12 show the addition of a hydraulically implemented active override 300. One can see why these systems need little added power: the spring is doing the lion's share of the work just as it did acting alone passively. The only extra force required is that needed to overcome friction in the system, the energy stored in the spring when displaced from its neutral set-point, and the inertia in the moving parts.
FIG. 13 shows a block diagram of the active-over-passive system described. The motion of the vessel is monitored by a motion reference unit (MRU). The motion at the over-boarding sheave and the adjustments necessary to compensate for this are computed in a computer or a programmable logic controller (PLC) from the data provided by the MRU. The PLC then directs hydraulic fluid to actuate the hydraulic cylinder in the appropriate direction. The actual motion is fed back to the PLC from a measuring device. The active portion of the system as described is implemented with a hydraulic cylinder but those skilled in the art will recognize other mechanisms could be used to add the necessary energy, such as, e.g., a motor driving a rack and pinion.
FIG. 14 depicts another shortcoming with gas-spring compensation systems, whether active or passive. The lift line carrying the payload being compensated traverses all the sheaves of the gas spring. This is true not just when compensating, but for the entire ascent and descent from the vessel to the final operating depth. At each sheave the rope or wire bends over that sheave causing wear. We refer to this as “inline compensation,” and all inline compensators are bend-over-sheave (BOS) multipliers. The lift line, whether wire or new synthetic fiber, may be three or more miles long in marine operations, for example, and cost in excess of $150,000; thus wear and deterioration of the rope is a serious matter even without considering the value of the payload connected to the vessel by this single thread. It is also difficult to monitor the condition of the rope over its entire length during routine operations.
For all of the aforementioned reasons there exists a need for a low power payload control apparatus and heave compensation systems (active or passive) and associated methods in which the heave-compensated load line is not required to traverse the sheaves of the gas-spring doing most of the work.